In a conventional method for forming a patterned layer such as a wiring (interconnection) layer to fabricate an LSI (Large-Scale Integrated circuit), a patterned SiO.sub.2 layer is formed on a Si substrate. A poly-Si layer is then deposited on the SiO.sub.2 layer by CVD, and an Al layer is formed on the poly-Si layer by evaporation or CVD. Then, the Al layer is etched partly by photolithography to have a predetermined pattern. Such a method for forming a wiring layer of an LSI is described on pages 85 to 100 of Nikkei Microdevice, December number, 1986, for example.
However, the conventional method for forming a patterned layer needs the complicated photolithography process including steps of coating, exposing and developing of resist. Further, such a photolithography process causes contamination of a fabricated wafer, so that a yield of products may reduce. Additionally, two kinds of etchants are required to etch the poly-Si layer and the Al layer.
Recently, a photo-induced selective CVD method has been used for forming a patterned layer. In the conventional method for forming a patterned layer by the photo-induced selective CVD, deposition of a CVD layer is suppressed selectively on some area of semiconductor substrate where a light having a predetermined wave length is irradiated to form a patterned CVD layer. This method has an excellent resolution in pattern transfer in comparison with a selective CVD method in which a CVD layer is deposited selectively on some area here a light is irradiated, because undesired deposition of a layer caused by a vapor decomposition is removed.
Such a conventional method for forming a patterned layer by the CVD is described in Japanese Patent Provisional Publication (Kokai) No. 62-116786 (Japanese Patent Application No. 60-254582), and on a page 592 of Extended Abstracts of the 36th Spring Meeting of the Japan Society of Applied Physics and Related Societies, Chiba, April, 2p-L-5, (1989, Spring Term). These methods use desorption of adsorbates composing a CVD layer from a surface of the semiconductor substrate, by irradiating infrared rays which resonate with a vibrational energy of bonds between the substrate and the adsorbates, or by irradiating vacuum ultraviolet rays which break the bonds between the substrate and the adsorbates.
According to the conventional method for forming a patterned layer by the photo-induced selective CVD, however, there are disadvantages as described below.
First, suppression of the CVD on irradiated area of the substrate is not sufficient, so that selectivity of growth of the CVD layer is not sufficient. The first reason is that deposition induced by thermal decomposition of the adsorbates due to the heating of the substrate by irradiating infrared rays occurs in addition with the desorption of the adsorbates, if infrared rays are used. The second reason is that photochemical CVD occurs in addition with desorption of the adsorbates.
Second, only limited kinds of light sources can be used in the method, because the light sources should be selected from those radiating infrared rays which resonate with the vibrational energy of bonds between the substrate and the adsorbates. Usually, more than two kinds of the adsorbates exist, so that many kinds of light sources each corresponding to each adsorbate to be resonated should be prepared. This makes cost high. On the other hand, if the ultraviolet rays are used for breaking the bonds between the substrate and the adsorbates, valence electrons in the adsorbates are excited from the bonding orbital to the anti-bonding orbital. This means that the initial state of the optical exciting and the final state thereof are determined, so that only a light having a wavelength corresponding to the energy equal to the gap energy of the initial and final states of the optical exciting is necessary to be used. If vacuum ultraviolet rays are used, ionization of the adsorbates occurs, so that the bonds between the substrate and the adsorbates are broken to contribute to the suppression of the CVD reaction. In this case, any light can be used so long as it has an energy larger than the threshold energy of ionization thereof, however, the suppress ion the CVD reaction by desorption of the adsorbates is not sufficient by the ionization due to the valence band excitation.
Third, it is difficult to obtain a patterned layer having sharp edges because of a light diffraction. The irradiation area of the light is defined by a profile of an opening in a mask which to be used for patterning, and the sharp edges can be obtained by avoiding the diffraction of the light to the non-irradiation area. The degree of the diffraction is linear to the wavelength of the light, so that the diffraction is much limited by using vacuum ultraviolet rays having a wavelength shorter than that of infrared rays, however it is still not sufficient for avoiding the diffraction.